This invention pertains to a novel method of fracturing subterranean coal formations as a means for stimulating the production of coalbed methane. A highly conductive proppant pack is emplaced in the fracture during the fracturing treatment that has a particle size gradient ranging from about 40/70 mesh at the leading tip of the fracture to about 20/40 mesh at the trailing base of the fracture. 2. Description of the Prior Art:
Coal is the most abundant fossil energy resource in the world. Its recoverable reserves amount to almost 100 quintillion Btu of energy, nearly 15 times the total energy content estimated for known reserves of petroleum. Petroleum Frontiers, Vol. 3, No. 4, pages 2-3 (1986), published by Petroleum Information Corporation. People have mined coal and used it for heat for centuries. However, it is within the recent past that coal has been recognized for being the origin and source for another hydrocarbon fuel, i.e., coalbed methane. Coalbed gas consists primarily of methane (e.g., 95 percent) but may also contain ethane, propane and higher homologs. The volume of coalbed methane is estimated to be about 400 trillion standard cubic feet (SCF) of gas-in-place. Most of this gas is adsorbed on coal seams buried at a depth of less than about 9000 feet (ft) from the surface, and almost half of it is coal seams buried less than about 3000 ft. This coal is generally too deep to mine but easily penetrated by a wellbore using conventional drilling techniques. Coalbeds are, therefore, reservoirs and source rocks for a huge amount of gas which can be produced, in part, through a wellbore. Methods of recovering the gas (i.e., coal degasification methods) are shown, for example, by U.S. Pat. Nos. 4,471,840, 4,391,327 and 4,301,875.
The U.S. Department of Energy, the U.S. Bureau of Mines and the Gas Research Institute have funded a substantial amount of research on coal degasification, and the results have been published in the open literature. In addition, periodic coalbed methane symposiums are held at the University of Alabama, and elsewhere, and the results are published as symposium proceedings. Many of the journal articles describe stimulation techniques used by the industry to enhance production of gas. Conventional hydraulic fracturing techniques are the most common. In hydraulic fracturing, a fracturing fluid (e.g., an aqueous gel or an aqueous foam) is injected through a wellbore and against the face of the formation at pump rates and pressure sufficient to hydraulically fracture the formation. Typically, a proppant (e.g., 20/40 mesh sand, sintered bauxite, and the like) is blended with the fracturing fluid and is carried by the fluid into the fracture. When the pump rate and pressure are released, the fractured formation closes or heals onto the emplaced proppant in the induced fracture and a permeable communication channel is thereby established from the tip of the pack of proppant back to the wellbore. The formation fluids flow through this communication channel to the wellbore and are withdrawn.
Some of the previous methods of fracturing coal are shown below.
U.S. Pat. No. 4,471,840 (Lassiter) The coal formation is fractured by injecting (a) a pad fluid, (b) a proppant-laden fracturing fluid, and (c) an overflush of a proppant-free fluid through a well and into a coal seam.
U.S. Pat. No. 4,566,539 (Perlman) The coal formation is fractured by injecting alternating slugs of a proppant-laden fracturing fluid and a acidizing fluid through a well and into the coal seam.
U.S. Pat. No. 4,665,990 (Perlman) Essentially the same technique is used as in the preceding patent, but the focus here is on the amount of fine mesh size proppant (e.g., 100 mesh sand) emplaced in the fracture relative to the vertical thickness of the coal seam and also upon the size of the casing/tubing used to complete the well.
U.S. Pat. No. 4,679,630 (Wyman) The coal seam is fractured by injecting fracturing fluid through perforations located above and/or below the coal seam. The fracture propagates through the adjacent formation and then into the coal seam. This technique allegedly reduces plugging by coal fines.
Major problems have been encountered during the fracturing treatment in the forms of (a) very high high pressure build-up during pumping of the proppant-laden fracturing fluid, and (b) an excessive number of screenouts. A "screen out" occurs when proppant bridges over the fracture and prevents further introduction of fracture fluid into the treatment zone and prematurely stops the treatment. The pressure build-up (i.e., the proppant induced pressure increase) can range from several hundred to several thousand pounds per square inch (PSI) during typical fracturing operations using 12/20 mesh sand proppant and aqueous gelled fracture fluids. Such pressure increases may exceed the pressure limitations of the pumping equipment on tubulars, resulting in a "pressure out", and the pressure increases require additional hydraulic horsepower (i.e., pumping capacity) to pump the fracture fluids at required fracturing pressure and flow rates. This, in turn, substantially increases the cost of the well. Attempts to counter the proppant induced pressure increases by reducing the concentration of proppant in the fracture fluid have been only partially successful, and at an increased cost per well for the larger volumes of fracture fluid. In addition to the cost, the prior fracturing techniques for coal degasification wells are plagued by a tendency to screen out during the treatment.
With prior art fracturing techniques, screen out rates of about 50 to 60 percent on coal degasification wells are not uncommon. The economic loss associated with screen outs is substantial.